The wellbore pressure of an oil and as well is usually contained by pressure control equipment having resilient internal seals that seal the wellbore opening around the wellbore tubing string. Snubbing is the process of attaching a length of pipe or tubing onto the wellbore tubing string and inserting or advancing the wellbore pipe string with the newly attached length of pipe into the wellbore when the wellbore is under pressure. Snubbing often requires significant downward force on the wellbore pipe string in order to push and advance the newly attached length of pipe through the internal seals of the wellborn pressure control equipment.
Special equipment called snubbing units are used during snubbing. A typical snubbing unit has a traveling slip-type spider and that moves upward and downward between upper and lower stationary frames. The upper stationary frame serves as a work platform. The lower stationary frame supports a stationary spider used to support the wellbore pipe string at a position above the wellbore pressure control equipment. The traveling frame is attached to and supported upon the extendable and retractable piston rods of large hydraulic jacks or cylinders mounted between the lower stationary frame and the traveling frame. Extension and retraction of the piston rods of these hydraulic cylinders moves the traveling frame vertically upward and downward with respect to the lower stationary frame. This upward and downward movement allows the traveling frame and the traveling spider to be extended upward through the opening in the workbasket and downward to a position approaching the lower stationary frame and stationary pipe gripping device. The traveling frame and hydraulic cylinder combination is sometimes called a snubbing jack and the cylinders and piston rods of the duplex snubbing jack can be sized as described to produce a desired snubbing load of force.
Whey snubbing a new length pipe onto a wellbore tubing string, the wellbore tubing string is supported by the stationary spider on the stationary frame and the piston rods of the hydraulic cylinders are extended to rare the traveling frame into the opening of the workbasket. The new length of pipe is their positioned to extend vertically downward from the workbasket where it is gripped by the traveling spider of the traveling frame and held in position over the wellbore pipe string. The traveling frame with the area length of pipe is then moved downward toward the wellbore pipe string by retraction of the piston rods of the hydraulic cylinders supporting the traveling frame to a position where the new length of pipe may be threadedly connected to the wellbore pipe string by means of a top drive, rotary, tongs, or other rotating equipment. When the new length of pipe is attached to the wellbore pipe string, the wellbore pipe string is released from the stationary spider with the traveling spider gripping the new length of pipe and the attached wellbore string. The tubular piston rods of the hydraulic cylinders supporting the traveling frame are then further retracted to push or snub the new length of pipe through the internal seals of the pressure control equipment to advance the wellbore pipe string into the wellbore. This process is reversed when a pipe segment is removed from the wellbore pipe string.
Because snubbing is done under wellbore pressure, the weight of the wellbore pipe string is often less than the force generated upon the wellbore pipe string by the wellbore pressure. In such a situation, the wellbore pressure may push the wellbore pipe string upward from the wellbore. To prevent the pipe string from being ejected from the wellbore due to the wellbore pressure, the traveling and stationary frames and their associated pipe gripping devices must be positioned to maintain firm control of both the wellbore pipe string and the new length of pipe being added to the wellbore pipe string.
One problem associated with typical snubbing units is that the new length of pipe being snubbed into the wellbore is unsupported along its length when the new length of pipe is forced downward into the wellbore against the upward wellbore pressure by retraction of the piston rods of the hydraulic cylinders. The compressive forces placed on the unsupported length of pipe during snubbing increases the risk that the new length of pipe will buckle as it is pushed through the internal seals of the pressure control equipment. To reduce the risk of buckling, only relatively short lengths of tubing are inserted on each cycle of the traveling frame. This increases the time and costs associated with a typical snubbing operation.
Another problem associated with typical snubbing units is time required for the travelling spider to move from the top of the piston stroke, the position where the piston rods of the hydraulic cylinders are extended and a new length of pipe is grabbed, to the bottom of the piston stroke, the position where the new length of pipe is released after it is pushed through the internal seals of the pressure control equipment, and then return to top of the piston stroke where another length of pipe may be grabbed. The time required for the travelling spider to fully cycle downward for snubbing a new length of pipe and then upward to grip another length of pipe also increases the time associated with snubbing operations.
Consequently, there is a need for a snubbing unit jack that will provide support for the length pipe being snubbed to minimize the risk of buckling. Reducing the risk of pipe buckling will reduce the risk of damage to the wellbore pressure control equipment as well as the snubbing unit itself, will reduce the risk of injury to workers performing the snubbing operations, and will consequently reduce the risk and cost of such snubbing operations.
There is also a need for a snubbing unit jack that will minimize the time associated with performing snubbing operations. Minimizing the time associated with such snubbing operations will correspondingly reduce the overall costs associated with such operations.